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You can get an internationalized message in code via Messages or directly in template (see how you can achieve this in freemarker template engine).
First you need a definition of supported languages in your conf/application.properties file. This is a simple comma separated list (whitespaces are omitted).

# Comma-separated list of supported languages
# First language specified is the default
application.languages = en, ro, de, ru, fr, es

The languages are one or two part ISO coded languages. Usually they resemble language or language and country.
Examples are en, de, en-US, en-CA and so on.

You can access the Languages object via Application.get().getLanguages().

The messages file name follows a convention. The convention is messages_LANGUAGE.properties or messages_LANGUAGE-COUNTRY.properties.

Some examples:

A messages file might look like:

pippo.welcome = Welcome!
pippo.greeting = Hello, my friend!
pippo.languageChoices = Language Choices
pippo.yourLanguageAndLocale = Your language is <b>{0}</b> and your locale is <b>{1}</b>.
pippo.theContextPath = The context path is <b>{0}</b>.
pippo.demonstrations = Demonstrations
pippo.unmatchedRoute = Unmatched Route
pippo.exceptionHandling = Exception Handling

Internally we use MessageFormat.format(text, values) to format the messages. Therefore all informations from MessageFormat do apply.

Bellow is an example how you can retrieve programmatically an internationalized message:

// send an internationalized message as response
GET("/i18n", routeContext -> {
	String message;

	String lang = routeContext.getParameter("lang").toString();
	if (lang == null) {
		message = getMessages().get("pippo.greeting", routeContext);
	} else {
		message = getMessages().get("pippo.greeting", lang);
	}

	routeContext.send(message);
});

Also, you can access the Messages object via Application.get().getMessages().